In-plane switching mode liquid crystal display device

ABSTRACT

An in-plane switching mode liquid crystal display device (LCD) is disclosed in which a first portion of one pixel region has alignment direction clockwise inclined relative to the extension direction of the data electrode, and a second portion of the pixel region has alignment direction counterclockwise inclined relative to the extension direction of the data electrode. This in-plane switching mode LCD has an improved viewing angle characteristics.

BACKGROUND OF THE INVENTION

[0001] A. Field of the Invention

[0002] The present invention relates to a liquid crystal display device,and more particularly to an in-plane switching (“IPS”) mode liquidcrystal display device.

[0003] B. Description of the Related Art

[0004] Recently, the thin film transistor liquid crystal display devices(TFT LCDs) have been used as display devices in such applications asportable televisions and notebook computers, but these TFT LCDs havesmall viewing angles.

[0005] In order to solve this problem, twisted nematic LCDs having, forexample, optical compensation films and multi-domains, have beenintroduced. In these LCDs, however, the color of the image is largelyshifted as the viewing angle direction increases.

[0006] For a wide viewing angle, the IPS mode LCD is disclosed, forexample, in JAPAN DISPLAY 92 page 547, Japanese Patent UnexaminedPublication No. 7-36058,Japanese Patent Unexamined Publication No.7-225388, and ASIA DISPLAY 95 page 707.

[0007]FIG. 1 is a view showing the operation of liquid crystal (LC)molecules of the conventional IPS mode LCD. FIG. 2 is a view showing thelong axis of the LC molecules shown in FIG. 1. As shown in the figures,a data electrode 8 and a common electrode 9 are aligned in Y axisdirection at an angle θ_(EL)=90° relative to the X axis direction whichis the extension direction of a gate bus line (not shown). θ_(E)=180°indicates an angle between the direction of electric field and X axisdirection. θ_(R) indicates an angle between the alignment direction andthe X axis direction, and angle θ_(R) is determined by a rubbing processin the range of 90° to 180°. θ_(rot) indicates a rotated angle of liquidcrystal molecules 35 by the electric field applied between the twoelectrodes.

[0008] When a voltage is not applied, liquid crystal molecules 35denoted by dotted line are aligned according to the alignment directionat angle θ_(R). When the voltage is applied, liquid crystal molecules 35rotate counterclockwise by angle θ_(rot). In the figures, liquid crystalmolecules 35 are denoted by solid line after the voltage is applied, anda positive type liquid crystal is used in which electric permittivity ofliquid crystal molecules in the long axis direction is larger than inthe short axis direction. On the other hand, when negative type liquidcrystal is used, the liquid crystal molecules are aligned perpendicularto the electric field direction.

[0009] However, in the conventional IPS mode LCD, grey level inversionis caused by birefringence of the liquid crystal molecules as shown inFIG. 3a. Further, as shown in FIG. 3b, a white color is shifted to blueand yellow colors respectively in A and B viewing directions shown inFIG. 1.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is to prevent a color shift inan in-plane switching mode liquid crystal display device.

[0011] Another object of the present invention is to prevent a greylevel inversion in an in-plane switching mode liquid crystal displaydevice.

[0012] Additional objects and advantages of the invention will be setforth in part in the description which follows, and in part will beobvious from the description, or may be learned by practice of theinvention. The objects and advantages of the invention will be realizedand attained by means of the elements and combinations particularlypointed out in the appended claims.

[0013] To achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the in-planeswitching mode liquid crystal display of the present inventioncomprises: a first substrate having a plurality of pixels, data andcommon electrodes over the first substrate and substantially parallelwith a reference direction, a first alignment layer over the data andcommon electrodes, wherein at least one of the plurality of pixels has aplurality of domains, and at least two adjacent domains of the pluralityof domains have alignment directions opposite one another with respectto the reference direction.

[0014] In another aspect the in-plane switching mode liquid crystaldisplay of the present invention comprises: a substrate having aplurality of pixels, data and common electrodes over the substrate andsubstantially parallel with a reference direction, an alignment layerover the data and common electrodes and having first and second domainsadjacent one another, each of the first and second domains having atleast one of the pluarity of pixels and an alignment direction oppositethat of the other domain with respect to the reference direction.

[0015] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

[0017] In the figures:

[0018]FIG. 1 is a view showing the operation of liquid crystal moleculesin the conventional IPS mode LCD;

[0019]FIG. 2 is a view showing the long axis of the LC molecules shownin FIG. 1;

[0020]FIGS. 3a and 3 b are views showing a contrast ratio and a colorshift of the conventional IPS mode LCD respectively;

[0021]FIGS. 4a and 4 b are plan and sectional views showing an IPS modeLCD according to the present invention;

[0022]FIG. 5 is a view showing the operation of liquid crystal moleculesin a first embodiment of the present invention;

[0023]FIG. 6 is a view showing a long axis of the LC molecules shown inFIG. 5;

[0024]FIGS. 7a and 7 b are views showing a viewing angle characteristicand a color shift of the present invention respectively;

[0025]FIG. 8 is a view showing a second embodiment of the presentinvention;

[0026]FIG. 9 is a view showing a third embodiment of the presentinvention; and

[0027]FIG. 10 is a view showing a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION

[0028] Reference will now be made in detail to the embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

[0029]FIGS. 4a and 4 b are plane and sectional views showing one pixelregion of the IPS LCD implemented according to the present invention,where FIG. 4b is a sectional view taken along line A-A′ of FIG. 4a. Asshown in these figures, a gate bus line 1 and a data bus line 2 arealigned to cross each other on a first substrate 10, defining a pixelregion. Although only one pixel region is shown in the figures, a liquidcrystal display device generally has a plurality of pixel regions.

[0030] At the cross of gate and data bus lines 1 and 2, a thin filmtransistor (TFT) is formed, which comprises a gate electrode 5, a gateinsulating layer 12, a semiconductor layer 15. an n+semiconductor layer16, a source electrode 6 and a drain electrode 7.

[0031] In the pixel region, a data electrode 8 and a common electrode 9are formed parallel to each other. Gate and common electrodes 5 and 9are connected electrically to gate and common bus lines 1 and 3respectively, and source and drain electrodes 6 and 7 are connectedelectrically to data bus line 2 and data electrode 8 respectively.

[0032] Gate and common electrodes 5 and 9 are formed by etching a metallayer deposited by sputtering a metal such as Al, Mo, Ta or Al alloy.Gate insulating layer 12 is formed by depositing silicon oxide orsilicon nitride on the surface of first substrate 10 which includes gateelectrode 5, common electrode 9, and gate bus line 1. Semiconductorlayer 15 is formed by etching an amorphous silicon layer deposited ongate insulating layer 12. n+ semiconductor layer 16 is formed bydepositing an n+ amorphous silicon layer or doping an n⁺ ion intopredetermined regions of the semiconductor layer 15. A passivation layer20 is formed thereon by depositing silicon oxide or silicon nitride.Data bus line 2, data electrode 8, source electrode 6, and drainelectrode 7 are formed thereon by depositing and etching a metal such asCr.

[0033] On passivation layer 20, a first alignment layer 23 a is formedby coating polyimide, polyamide or photo alignment materials. Thepolyimide or polyamide alignment layer is rubbed to impart an alignmentdirection. On the other hand, the photo-alignment layer such aspolyvinylcinnamate or polysiloxane based materials may also be used andbe exposed to an ultraviolet light to impart an alignment direction.

[0034] On a second substrate 11, a black matrix 28 is formed to preventa leakage of light through the regions of the TFT and gate, data andcommon bus lines 1, 2 and 3. Black matrix 28 is formed by depositing andpatterning a Cr layer, a CrOx layer, or a black resin layer. A colorfilter layer 29 is formed thereon, which has one of R, G and B colorfilter elements (not shown) in each of the pixel regions. A secondalignment layer 23 b is formed thereon by coating polyimide orpolyamide, or photo-alignment materials such as polyvinylcinnamate orpolysiloxane based materials.

[0035] A liquid crystal layer 30 is formed by injecting liquid crystalbetween the two substrates.

[0036]FIG. 5 is a view showing the operation of liquid crystal (LC)molecules in a first embodiment of the present invention. FIG. 6 is aview showing the long axis of the LC molecules shown in FIG. 5. Althoughnot illustrated in FIGS. 4a and 4 b, this embodiment is characterized inthat one pixel region is divided into a first domain I and a seconddomain II which are arranged parallel to the extension direction of thetwo electrodes. In FIG. 5, only a pair of electrodes 8 and 9 isillustrated for convenience of explanation.

[0037] As shown in FIGS. 5 and 6, data electrode 8 and common electrode9 are i aligned in Y axis direction at an angle θ_(EL)=90° relative tothe X axis direction that is the extension direction of gate bus line 1shown in FIG. 4a. θ _(E)=180° indicates an angle between the electricfield and X axis directions. θ_(R1) and θ_(R2) indicate angles ofinclination between the Y axis direction and the first and secondalignment directions respectively. θ_(rot1) and θ_(rot2) indicate therotated angles of LC molecules 35 and 36 in the first and second domainsrespectively. In this embodiment, it is preferable that the LC molecules35 and 36 in the two domains are aligned symmetrically relative to theextension directions of electrodes 8 and 9.

[0038] When a voltage is not applied to the device, LC molecules 35 and36 are aligned according to the alignment directions of the first andsecond domains. A positive type liquid crystal is used in thisembodiment, and therefore the two alignment directions are determined inthe ranges of 0° to 45°. On the other hand, when a negative type liquidcrystal is used, the two alignment directions are determined in theranges of 45° to 90°. In order to obtain an untwisted mode LCD,alignment directions of the second alignment layer are determined to bethe same as alignment directions of the first alignment layer. On theother hand, the alignment directions of the second alignment layer aredetermined to be perpendicular to the alignment directions of the firstalignment layer to obtain a twisted mode LCD.

[0039] When a voltage is applied to the device, liquid crystal molecules35 and 36 in the first and second domains are rotated in oppositedirections to each other by angles θ_(rot1) and θ_(rot2) respectively,being symmetrical relative to the extension direction of data and commonelectrode 8 and 9.

[0040] In this embodiment according to the present invention, the greylevel inversion and the color shift can be prevented by the rotation ofliquid crystal molecules in opposite directions, improving the viewingangle characteristics.

[0041]FIGS. 3a and 3 b are views showing the contrast ratio and thecolor shift of the conventional IPS mode LCD respectively, where greylevel inversion regions are denoted by the shaded areas. FIG. 7a andFIG. 7b are views showing the contrast ratio and the color shift in thepresent invention respectively. As shown in FIG. 3a, the conventionalIPS mode LCD has grey level inversion regions at polar viewing angle 60°and at azimuthal viewing angles between 90° and 180° and between 270°and 0°, while there is no grey level inversion in the LCD implementedaccording to the present invention at polar viewing angle 60° as shownin FIG. 7a.

[0042] As shown in FIG. 3b, the conventional IPS mode LCD has a widewhite color shift, while the present invention has a narrow white colorshift as shown in FIG. 7b.

[0043]FIG. 8 is a view showing a second embodiment, wherein one pixelregion is divided into two domains which are arranged perpendicular tothe extension direction of electrodes 8 and 9.

[0044]FIG. 9 is a view showing a third embodiment, wherein one pixelregion is divided into four domains. Alignment directions in the twoupper domains are inclined in opposite directions to each other relativeto the extension directions of data and common electrodes 8 and 9.Alignment directions in the two lower domains are also inclined inopposite directions to each other relative to the extension directions.In this embodiment, alignment directions in two left domains and in tworight domains are inclined in opposite directions to each other,compensating for grey level inversions and color shifts of each other.

[0045] In the embodiments according to the present invention, one pixelregion is divided into a plurality of domains. Generally, it ispreferable that half of the plural domains have alignment directionsclockwise inclined relative to the extension directions of the data andcommon electrodes, and the other half of the plural domains havealignment directions counterclockwise inclined relative to the extensiondirections of the data and common electrodes, so that the alignmentdirections of the half of the plural domains are symmetrical to those ofthe other half of the plural domains relative to the extensiondirections of the data and common electrodes.

[0046]FIG. 10 is a view showing a fourth embodiment, wherein thealignment directions in adjacent two pixel regions are inclined insymmetrically opposing directions to each other relative to theextension directions of two electrodes 8 and 9. Further, the presentinvention can provide an IPS mode LCD in which adjacent plural pixelregions of more than two, for example four adjacent pixel regions, arealigned symmetrically relative to the extension directions of data andcommon electrodes 8 and 9, so that they are rotated in oppositedirections to each other. In this case, it is preferable that half ofthe plural pixel regions have alignment directions inclined clockwiserelative to the data and common electrodes, and the other half of theplural pixel regions have alignment directions inclined counterclockwiserelative to the data and common electrodes.

[0047] In the IPS mode LCD according to the present invention, becauseeach LC molecules in plural domains or in adjacent plural pixel regionsare symmetrically rotated in opposite directions so as to compensate forthe angular dependence of each other, grey level inversion and colorshift are eliminated to improve the viewing angle characteristics.

[0048] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the in-plane liquid crystaldisplay device of the present invention and in construction of thisdevice without departing from the scope or spirit of the invention.

[0049] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed:
 1. An in-plane switching mode liquid crystal displaydevice, comprising: a first substrate having a plurality of pixels; dataand common electrodes over the first substrate and substantiallyparallel with a reference direction; and a first alignment layer overthe data and common electrodes, wherein at least one of said pluralityof pixels has a plurality of domains, and at least two adjacent domainsof said plurality of domains have alignment directions opposite oneanother with respect to said reference direction.
 2. The deviceaccording to claim 1, further comprising: a second substrate facing thefirst substrate; and a second alignment layer over the second substrate.3. The device according to claim 1, wherein said first alignment layerincludes a material selected from the group consisting of polyimide,polyamide, polyvinylcinnamate and polysiloxane based materials.
 4. Thedevice according to claim 1, wherein said reference direction includes alongitudinal direction of one of said data and common electrodes.
 5. Thedevice according to claim 2, wherein said second alignment layer has analignment direction parallel with an alignment direction of said firstalignment layer.
 6. The device according to claim 2, wherein said secondalignment layer has an alignment direction perpendicular to an alignmentdirection of said first alignment layer.
 7. The device according toclaim 2, wherein said second alignment layer includes a materialselected from the group consisting of polyimide, polyamide,polyvinylcinnamate and polysiloxane based materials.
 8. An in-planeswitching mode liquid crystal display device comprising: a substratehaving a plurality of pixels; data and common electrodes over thesubstrate and substantially parallel with a reference direction; and analignment layer over the data and common electrodes and having first andsecond domains adjacent one another, each of said first and seconddomains having at least one of said pluarily of pixels and an alignmentdirection opposite that of the other domain with respect to saidreference direction.
 9. The device according to claim 8, wherein saidfirst and second domains have an equal number of pixels.
 10. The deviceaccording to claim 8, wherein the alignment directions of said first andsecond domains are symmetrical to one another with respect to saidreference direction.
 11. The device according to claim 8, wherein saidfirst and second domains each have one pixel.
 12. The device accordingto claim 8, wherein the first and second domains each have two pixels.13. The device according to claim 8, wherein said reference directionincludes a longitudinal direction of one of said data and commonelectrodes.
 14. An in-plane switching mode liquid crystal display devicecomprising: a substrate; data and common electrodes over the substrateand substantially parallel with a reference direction; and at least onealignment layer over the data and common electrodes, wherein a surfaceof said alignment layer has first and second domains adjacent oneanother, and said first and second domains have alignment directionsopposite one another with respect to said reference direction.
 15. Thedevice according to claim 14, wherein said reference direction includesa longitudinal direction of one of said data and common electrodes.